1. Field of the Invention
This invention relates generally to transporting a thin metal sheet or strip and, more particularly, to a method and apparatus for maintaining a transverse tension on a thin metal web without contacting the surface of the web, while at the same time the web is being transported longitudinally.
2. Description of the Background
In the production of modern color television picture tubes, an important component is the so-called shadow mask. This shadow mask causes the appropriate electron beam to fall on the proper color phosphor coated on the inner surface of the picture tube. The shadow mask is formed as a metal sheet that has a vast number of apertures formed therein, which apertures may be in the form of circular holes or slots. There is also known for use a grill mask that has a large number of closely spaced slits. Typically, a shadow mask is formed by using an etching process that involves coating a metal sheet with a photo-resist material, exposing the photo-resist material through a mask, and then etching the resultant pattern into the metal sheet. The metal sheets used for shadow masks are typically steel and are known to be made around 0.10 mm thick, although other thicknesses are available, such as 0.30 mm and thicker. The thinness of the material is generally limited by the difficulties presented in handling such thin materials.
In working with such materials it is quite difficult to prevent damage to the shadow mask as it is being operated upon to produce the desired aperture pattern. Problems are encountered with the material wrinkling and in maintaining tracking of the material when it is in the form of a continuous web. Continuous webs of this metal material are typically used in manufacturing such shadow masks, because at the end of the etching process the masks can be cut from the continuous web relatively easily. To improve the handling of this metal material it has been proposed to provide a support layer to add rigidity during transporting of the material, and such a system is disclosed in U.S. Pat. No. 4,755,257. The problem with this approach is that the support layer cannot be used indefinitely and must be periodically replaced, so that this increases the cost of the resultant shadow masks. Moreover, by using a support layer only one side of the web can be etched to form the apertures, which is not always a desirable approach in performing an etching process.